The present invention relates to a magnetic head control system in the magnetic disk device of an index servo system.
In a low-cost, compact, hard magnetic disk device (e.g., 5" hard disk, 610 tracks, 25-Mbyte capacity, 15-kbpi), a stepping motor is often used as a drive source for executing the magnetic head's seek operation. Positioning control of a magnetic head by a stepping motor involves the problem of positional error due to low angle precision of the stepping motor, and changes in ambient temperature.
The former problem can be resolved by decreasing the track width in comparison with the track interval in order to prevent interference between adjacent tracks. Thus, errors in accessing an adjacent track are prevented by increasing the track interval. However, the latter problem cannot be resolved even if the track width is decreased, since the position of the magnetic head changes in read and write modes depending upon changes in ambient temperature. In view of this, a servo system has been adopted. In this system, servo data stored in a magnetic recording medium (to be referred to as a disk hereinafter) is used to improve positioning precision of the magnetic head in its seek operation.
Although various servo systems have been proposed, an index servo system is normally adopted in a magnetic disk device of this type. In the index servo system, servo data is recorded in part of a first sector of each disk track, and positioning control is performed upon each rotation of the disk.
In a seek operation according to the index servo system, the magnetic head is moved to a target track of the disk by a stepping motor. The magnetic head remains static until the positioning system stops oscillating. When the disk rotates again and a servo sector comes to the magnetic head position, the head reads servo data. The magnetic head is moved by the stepping motor in accordance with the movement control amount specified in the servo data. When the positioning system stops oscillating, the seek operation ends. The magnetic head is thus positioned on the target track to a high degree of precision.
However, in the index servo seek operation as described above, the seek operation is prolonged as compared to the case wherein a servo system is not adopted. When the seek operation is long, i.e., when the number of tracks to be jumped is large, the moving time to a target track itself is long and the time required for positioning is not a critical problem. However, in a short seek operation, the moving time to a target track may become shorter than the positioning time (i.e., time required for the magnetic head to be securely positioned on the target track; index servo data is read an average of three times in order to perform such positioning). In such a case, the ratio of positioning time to the overall average seek operation time (i.e., the average time required up to final positioning including moving time to the target track, irrespective of the number of tracks to be jumped) increases, and an improvement in this respect is desired.